Microcomputers are sophisticated, general purpose logic devices which can be programmed to perform a wide variety of useful control functions in industrial and communications equipment, large-scale and medium-scale computer peripheral and terminal hardware, automobiles and other transportation media, amusement and educational devices, and the like.
Microcomputers are being used increasingly to control serial data communications between data processing terminals, modems, and processors. Serial data communications between terminals or modems primarily utilizes NRZ (non-return-to-zero) encoding, whereas serial communications between processors primarily utilizes biphase encoding. Particularly, as distributed processing systems become increasingly important, such as in the automotive environment, it also becomes increasingly important to provide a microcomputer with the capability of handling the highly reliable biphase format, which can tolerate a much greater mismatch between transmitter and receiver clocks.
As the unit cost of microcomputers decreases, these devices are becoming more and more attractive for controlling serial data communications of all types. It would be highly desirable to provide a microcomputer with the capability of utilizing both NRZ and biphase protocols. However, in designing a microcomputer which can be sold at a competitive price it is mandatory that the on-chip logic be minimized to the greatest extent possible, since yields are inversely proportional to the chip size. There is a need therefore for a microcomputer serial I/O communication logic which can accommodate both NRZ and biphase encoding, yet which requires a minimum of logic to be implemented.